sub or main ECU for NOS must be necessary?
It depends on the system.
For a small shot, no ECU alteration is needed, as the stock ECU and fuel system can usually handle the required fuel demands. Too much N2O and not enough fuel can cause a lean condition and engine damage. Most systems have an integrated solution to this problem.
Wet systems inject both Fuel and N2O through the injectors or dedicated ports into each cylinder with dedicated fuel and nitrous solenoids, when the system engages, the system will add additional fuel and N2O to the combustion chambers. examples include most kits made by NOS, Edelbrock, or the NX systems. Some kits require little plumbing, as they have piggyback nozzles for the injectors, and solenoids are contained in a new fuel rail
On Dry fog systems (the kind that is injected into Intake
tube) a means of supplimenting fuel is needed, either bigger injectors, FPR or an AFC is needed to increase fuel flow, otherwise you fuel mixture leans out, and your motor blows up.
Most computerized nitrous systems (Zex, Venom VCN systems) have the needed hardware to suppliment the fuel by altering sensor maps to cause a rich condition, and are very reliable. some even add fuel through an independent solenoid, or by changing the injector pulse width.
If all else fails, ask the vendor, they usually know how each system handles fuel enrichment, and which is best for a given application. if installed properly, almost every system available can be used safely.